The present invention relates to a heat treatment apparatus for applying a heat treatment to an object such as a semiconductor wafer.
Important steps in the fabrication of a semiconductor wafer are heat treatment steps. Examples of such heat treatments include oxide layer formation, dopant diffusion, annealing, and chemical vapor deposition (CVD). Recently, vertical heat treatment apparatuses have come to be used more than horizontal furnaces, because they have advantages such as not inducing ingress of the atmospheric air.
A heat treatment apparatus requires a region for a means of transferring wafers into a wafer boat and for conveying a wafer vessel (hereinafter called a "carrier") that accommodates a number of wafers, such as a maximum number of 25 wafers, to and from the outside of the apparatus, and it also requires auxiliary equipment such as a storage rack for accommodating the carrier and a carrier transfer means. These must be accommodated in as small a space as possible, and the wafers and carrier must also be moved efficiently.
Prior art vertical heat treatment apparatuses have been constructed with these requirements in mind, such as that described in U.S. Pat. No. 5,181,819 to K. Sakala et al. That is, in this apparatus, carrier stages for conveying carriers to and from the outside of the apparatus is arranged along the front side of the entire apparatus, a wafer delivery section is provided in such a manner as to face the inner side of these carrier stages with a movement path for a carrier transfer mechanism therebetween, and also a carrier accommodation rack of, for example, four rows by two columns is arranged above the wafer delivery section in such a manner as to accommodate a maximum of eight carriers.
One heat treatment unit is arranged in the rear of the wafer delivery section, and the heat treatment unit is provided with a vertical heat treatment furnace, a boat elevator that loads and unloads a wafer boat with respect to the heat treatment furnace, and a wafer transfer mechanism that conveys the wafers between the wafer boat and the wafer delivery section.
In the above described prior art vertical heat treatment apparatus, two carriers each containing 25, for example, wafers are first placed on the carrier stages, and the carriers are turned sideways by a sideway-tipping mechanism. The carrier transfer mechanism is free to move forward and backward, and to the left and right, and is also free to rotate about a vertical axis, and it operates in such a manner that it conveys the two carriers into the wafer delivery section or the carrier accommodation rack. After a carrier has been transferred to the wafer delivery section, the wafer transfer mechanism sequentially transfers the wafers from within that carrier into a wafer boat, and then the wafer boat supporting the wafers is loaded into the heat treatment furnace.
The above described carrier accommodation rack has the function of operating as a buffer means for temporarily accommodating carriers containing product wafers, but otherwise it also has the function of accommodating carriers containing test wafers, carriers containing defective wafers used as supplementary waters, and carriers containing dummy wafers.
Test wafers are wafers that are subjected to the heat treatment together with the product wafers, then are separated from the product wafers and are sent on to a step that evaluates that heat treatment. Supplementary wafers are extra wafers that are added when there are less than the required number of product wafers, in order to prevent the changes in the gas flow within the heat treatment furnace and the resultant loss of uniformity in the heat treatment that would occur otherwise. Dummy wafers are wafers that are placed in the uppermost and lowermost positions in the wafer boat, with the objective of preventing product wafers from being subjected to the unevenness in temperature that occurs at these uppermost and lowermost positions.
Since the timing at which the carriers are conveyed between the heat treatment apparatus and the outside of the apparatus usually differs from the timing at which the heat treatment starts and ends, it is necessary to ensure that there are as many of the carriers accommodating the product wafers in the carrier accommodation rack as possible, in order to facilitate the carrier conveying and the heat treatment, and prevent any deterioration of the throughput.
In order to ensure a supply of wafers other than the product wafers, a minimum of one carrier accommodating test wafers and a minimum of, for example, two carriers accommodating dummy wafers are necessary, and thus the number of carriers accommodating supplementary wafers is limited in practice. It is preferable to have available a number of supplementary wafers that is one less than the number of product wafers that can be placed in the wafer boat. However, with a prior art apparatus, since there are so few carriers accommodating supplementary wafers, they are insufficient when the heat treatment is to be performed on only a small number of wafers (and thus more supplementary wafers are to be loaded in the wafer boat than the few product wafers, in order to ensure the uniformity of the heat treatment), and the degrees of freedom of the apparatus are reduced.
On the other hand, the heat treatment apparatus is provided with components such as a gas supply unit and exhaust unit connected to the heat treatment unit, and the entire apparatus should be made as compact as possible. In addition, the carrier stages and carrier accommodation rack are positioned in the expensive cleanroom facilities, and also a large number of heat treatment apparatuses are installed in actual fabrication works, and thus there is demand to make the area occupied by this equipment as small as possible. The above described heat treatment apparatus was designed with these requirements in mind, but, if the number of rows or columns in the carrier accommodation rack should increase, the entire apparatus will expand either horizontally or vertically by an amount equivalent to the size of the carriers. Therefore, it is not possible in practice to use a measure that simply increases the numbers of rows or columns of carriers.
Further problems with the above described apparatus include the high cost of the carrier transfer mechanism itself, and also its low movement efficiency in proportion to the large movement area that it occupice and its bad layout efficiency from the cost point of view.